The present invention relates generally to polarimetric instrumentation and associated methods for measuring the state of polarization of a light beam, and more particularly to a novel interferometric polarization interrogating filter assembly and method for measuring the spatially-varying, complete state of polarization across an image comprised of one or more optical wavefronts in a no-moving-parts, instantaneous manner.
A single optical wavefront or a plurality of such wavefronts, i.e., a wavefront ensemble, may carry with it information pertaining to the state of polarization, spectral content, radiance distribution, coherence and statistical composition. Polarization encoding of the wavefront ensemble occurs when light interacts with material media. By decoding the spatially-varying state of polarization across a partially-polarized image, it is possible to acquire additional information beyond the conventional irradiance image that enables one to discern heretofore-indiscernible characteristics of the original object.
There is a great diversity of naturally occurring and human-engineered polarization encoding schemes or xe2x80x9cpolarization signaturesxe2x80x9d in the physical world.
In astronomy, for example, it has been found that certain preferred states of polarization result when celestial dust grains scatter light from nearby stars. Some scientists speculate that such preferences in the state of polarization could help to explain why most living organisms show an overwhelming preference toward right-handed sugars and left-handed amino acids in the building of their cellular structure. See, e.g., Schneider, xe2x80x9cPolarized Life, Astronomers Probe Orion to Answer one of Life""s Mysteriesxe2x80x9d, Scientific American, Vol. 279, No. 4, October 1998, p. 24.
In the field of marine biology, it has been shown that certain cephalopods are sensitive to the azimuth of linearly polarized light by virtue of the orthogonal structure of photoreceptors in their retina. It has been further demonstrated that cuttlefish, i.e., Sepia officinalis L., communicate by using polarized light. See, e.g., Shashar, Rutledge and Cronin, xe2x80x9cPolarization Vision in Cuttlefishxe2x80x94A Concealed Communication Channelxe2x80x9d, The Journal of Experimental Biology, Vol. 199, No. 9, September 1996, pp. 2077-2084.
Just as prolific, in the field of entomology, it has been shown that certain ants and bees use polarized sky light as a navigational compass. See, e.g., Wehner, xe2x80x9cPolarized-Light Navigation by Insectsxe2x80x9d, Scientific American, Vol.235, No.1, July 1976, pp. 106-115.
In the semiconductor industry, ellipsometry is used to measure film thickness, refractive index and other material properties. A light beam with a controlled state of polarization whose spectral content is well defined is made to illuminate a test surface. By measuring the change in the state of polarization of the reflected beam, certain information concerning mechanical and material properties of the test surface can be inferred. See, e.g., Tompkins, A User""s Guide to Ellipsometry, Academic Press, New York, 1993.
Terrestrial resource mapping and surveillance investigations conducted via satellite are growing fields of study. Contrast enhanced polarization images of objects on the ground, as viewed from a low-flying satellite, can provide additional information to definitively identify those objects.
In fiber-optic components and systems employed in the telecommunications industry, signal polarization plays a crucial role. Accordingly, it is desirable to measure the state of polarization of a time-varying signal exiting from a single-mode fiber in an instantaneous manner.
In the military, polarization of light may be used to identify vehicles with friendly or hostile intent under less-than-ideal viewing conditions. Aircraft, for example, have quasi-specularly reflecting surfaces, which give rise to unique polarization signatures. See, e.g., Duggin, xe2x80x9cImaging Polarimetry in Scene Element Discriminatiornxe2x80x9d, in Goldstein and Chenault (eds.), xe2x80x9cPolarization: Measurement, Analysis and Remote Sensing IIxe2x80x9d, Proc. Soc. Photo-Opt. Instrum. Eng., Vol. 3754, July 1999, pp. 108-117.
Traditional approaches for measuring the state of polarization of an arbitrary wavefront ensemble include (1) discrete time-sequential, (2) continuous time-sequential, i.e., polarization-modulation, (3) division-of-amplitude, (4) division-of-aperture, and (5) interferometric methods. See, e.g., Hauge, xe2x80x9cSurvey of Methods for the Complete Determination of a State of Polarizationxe2x80x9d, Proc. Soc. Photo-Opt. Instrum. Eng., Vol. 88, 1976, pp. 3-10 and Hauge, xe2x80x9cRecent Developments in Instrumentation in Ellipsometryxe2x80x9d, Surface Science, Vol. 96, Nos. 1-3, June 1980, pp. 108-140.
Whereas all of these approaches have been utilized in the past for measuring the complete state of polarization in a point context, few are amenable to measuring the complete state of polarization across an image. Difficulties often arise in practice that limit the general usefulness of such methods for imaging polarimetry applications.
Arguably, the most common approach for measuring the spatially-varying state of polarization across an image employs a discrete or continuous time-sequential measurement modality in which a certain amount of time is required in between each step of the measurement process.
In the discrete time-sequential case, a set of polarization interrogating elements, i.e., polarizers, retarders or combinations thereof, are inserted one-by-one into the wavefront ensemble of interest and image acquisition is repeated for each element of the set. At least four separate elements and their corresponding images are required to compute the complete state of polarization across a partially-polarized image.
Walraven is generally accredited for being the first investigator to employ discrete time-sequential measurement methods using a 35 mm film-based camera. In his work, the transmission axis of a linear polarizer was manually oriented at 0, 45, 90 and 135 degrees respectively between sequential exposures of the same scene. See, e.g., Walraven, xe2x80x9cPolarization Imageryxe2x80x9d, Optical Engineering, Vol. 20, No. 1, January 1981, pp. 14-18. Although his work lacked determinacy with respect to the handedness of the polarization state across the image and his scene selection was severely restricted to static objects due to the time-sequential nature of his measurements, Walraven was successful in pointing out the importance of polarization image content.
In the continuous time-sequential case, one or more polarization interrogating elements are rotated in a continuous manner and the complete state of polarization is computed via Fourier analysis of the polarization-modulated signal. See, e.g., Berry, Gabrielse and Livingston, xe2x80x9cMeasurement of the Stokes Parameters of Lightxe2x80x9d, Applied Optics, Vol. 16, No. 12, December 1977, pp. 3200-3205.
As described by Aspnes in U.S. Pat. No. 3,985,447 and elsewhere by Azzam and others, conventional ellipsometric instrumentation frequently employs a continuous time-sequential measurement approach in a point context. Accordingly, in order to measure spatial variations in surface properties the point is methodically scanned across the surface and an xe2x80x9cimagexe2x80x9d of the desired property is constructed by mathematically xe2x80x9cstitchingxe2x80x9d the data together. Such scanning methods and numerical processing techniques are inherently time consuming for high-resolution images.
In polarization component metrology, Chipman has successfully employed continuous time-sequential measurement methods to compute the spatially-varying polarimetric properties of various optical components. See, e.g., Chipman and Somsin, xe2x80x9cMueller Matrix Imaging Polarimetry: An Overviewxe2x80x9d, in Yoshizawa and Yokota (eds.), xe2x80x9cPolarization Analysis and Applications to Device Technologyxe2x80x9d, Proc. Soc. Photo-Opt. Instrum. Eng., Vol. 2873, June 1996, pp. 5-12.
More often than not, spatially-varying polarimetric properties of optical components do not vary as a function of time. In such cases, a time-sequential measurement approach is adequate for computing the polarimetric parameters of interest.
Both discrete and continuous time-sequential measurement methods allow for the complete measurement of the state of polarization in a point or imaging context. However, the nature of these approaches precludes them from being used for measuring wavefront ensembles whose polarization content changes rapidly in time. Although various electro-optic and magneto-optic means have been employed for speeding up the measurement process, time-sequential approaches are generally not applicable to the measurement of time-varying systems.
Division-of-amplitude is an alternative class of polarimeter instrumentation that allows for the complete measurement of the state of polarization in a no-moving-parts, instantaneous manner. However, such an approach is not amenable to an imaging context. As the name implies, division-of-amplitude instruments necessarily employ a splitting or dividing means whereby the incident wavefront ensemble under test is broken up into two or more beams. Each beam is then separately interrogated and detected in a simultaneous manner. For example, in U.S. Pat. No. 5,073,025, Brooks has utilized several non-polarizing beamsplitter cubes to divide an incident laser beam into six separate beams each with their own interrogating element and detector array. When carefully registered, the data comprising the six detector arrays enable one to compute a mapping of the state of polarization across the beam. Although this method has proven to be successful for testing well-collimated laser beams it would not be an acceptable approach for an imaging system of any useful angular field. A laser beam is often highly collimated making the registration process between the array detectors a simple matter. In an imaging context, over any practical angular field it is exceeding difficult to keep the detector arrays in precise registration across the field.
Azzam has made several contributions to division-of-amplitude point polarimeter instrumentation. Most notable are his four-detector photopolarimeter, U.S. Pat. No. 4,681,450, and diffraction-grating photopolarimeter, U.S. Pat. No. 5,337,146. Both approaches are quite amenable to a well-collimated input beam. However, neither affords any real practical use in imaging polarimeter instrumentation.
Apart from the difficulty of multiple detector array registration, in a division-of-amplitude imaging polarimeter there is great redundancy of imaging system components and electronic circuitry required. Such redundancy can be expensive, fragile in nature and restrictive in certain applications. For example, in space-borne imaging polarimeter instrumentation such an approach necessarily increases on-board electrical power consumption.
Division-of-aperture allows the instantaneous measurement of the complete state of polarization in both quasi-point and imaging applications. Collett in U.S. Pat. No. 4,158,506 describes how a xe2x80x9cpolarizer assemblyxe2x80x9d comprised of a discrete array of six polarization interrogating elements positioned in front of a xe2x80x9csix element optical detector assemblyxe2x80x9d allows the complete state of polarization to be determined for laser pulses in an instantaneous manner. Taking this a step further, in U.S. Pat. No. 5,416,324, Chun has superposed a discrete micro-array of polarization interrogating elements onto the detector array surface so that all of the polarization information can be acquired across an image in an instantaneous manner. This measurement scheme requires extremely precise registration between the discrete elements comprising the polarization interrogating array and the picture elements, i.e., pixels, comprising the detector array. Such registration becomes exceedingly difficult as the pixels become smaller in size and larger in number as in the case of modern digital imaging instrumentation. Furthermore, such an approach is detector-specific in that the array of polarization interrogating filter elements can only be used with that detector geometry for which it was specifically designed.
It is commonly known that interferometers provide extremely sensitive means for determining a measurand of interest. It is this same heightened sensitivity that makes them susceptible to temperature, vibration and other deleterious effects that may have contributed to the lack of interest in employing interferometric methods for polarization measurement. As a result, interferometric approaches to polarization measurement are the least commonly employed among the various measurement schemes.
In an unusual point-polarimeter embodiment, Korth and Schedewie describe an interferometric approach for measuring the state of polarization in their U.S. Pat. No. 4,310,247. However, their approach is not applicable to an imaging context where the state of polarization can vary from point-to-point across the image. Nor is it possible to compute, from the interference pattern generated, the Stokes parameters as a function of position across the incident wavefront ensemble. The apparatus described by Korth and Schedewie does not provide a unique mapping. For example, one cannot differentiate between linear states of horizontal and vertical orientation. Both states result in the same null condition of zero fringe visibility.
Ohtsuka and Oka have described a means for mapping the spatially-varying state of polarization across an image using an interferometric method. See, e.g., Ohtsuka and Oka, xe2x80x9cContour Mapping of the Spatio-Temporal State of Polarization of Lightxe2x80x9d, Applied Optics, Vol. 33, No. 13, May 1994, pp. 2633-2636. However, their approach makes use of a non-common-path Mach-Zehnder interferometer that is highly sensitive to temperature and vibration effects to rendering it difficult to use outside of a controlled laboratory environment. In addition, their instrument requires the wavefront ensemble of interest to have a high degree of temporal and spatial coherence thus limiting the usefulness of the approach to quasi-monochromatic, well-collimated, coherent, wave fields.
Accordingly, there is a pronounced need for a new method of polarization measurement that is (1) amenable to an imaging context, and (2) allows instantaneous measurement of the spatially-varying, complete state of polarization across an image in a no-moving-parts, easily-deployed, robust manner.
The aforementioned need is fulfilled in the present invention by providing (1) a novel method by which polarization information contained within an arbitrary wavefront ensemble is uniquely encoded within an interference pattern, (2) a novel interferometric polarization interrogating filter assembly that produces a polarization-encoded interference pattern representative of the spatially-varying, complete state of polarization across the ensemble, and (3) a novel mathematical reconstruction algorithm for decoding the polarimetric parameters of interest from the electronic image produced by detector array discretization of the polarization-encoded interference pattern.
Novelty of the method described herein resides in the interferometric approach to polarization measurement whereby the spatially-varying, complete state of polarization across an image comprised of a plurality of optical wavefronts is uniquely mapped to irradiance variations in an interference pattern. This mapping or polarization-encoding process produces a unique irradiance fringe system for any input state of polarization. Local variations in the state of s polarization across an image result in corresponding local variations in the irradiance fringe system.
The new method consists of a novel combination of three separate operations. The first operation employs a first retardation-gradient encoding scheme across the incident wavefront ensemble of interest. The second operation employs a second retardation-gradient encoding scheme whose direction is orthogonal to the first. The third operation employs a polarizer to preferentially pass certain components of the now doubly retardation-encoded wavefront ensemble to facilitate the formation of a polarization-encoded interference pattern.
A preferred embodiment of the novel method is realized by a novel interferometric polarization interrogating filter assembly herein referred to as an Ortho-Babinet Polarization Interrogating (xe2x80x9cOBPIxe2x80x9d) filter. During use, an ensemble of electromagnetic wavefronts comprising a partially-polarized image is directed through the OBPI filter assembly. The resulting polarization-encoded image exiting the filter consists of an interference pattern superposed over the original input image. Only the polarized portion of the partially-polarized image gives rise to interference fringes. The unpolarized portion of the input image passes through the OBPI filter assembly substantially unaffected. The structure of the interference pattern contained within the polarization-encoded output image allows the spatially-varying, complete state of polarization to be determined across the input image in a point-by-point, no-moving-parts, instantaneous manner thereby enhancing the ability to discern heretofore-indiscernible characteristics of the original object.
The OBPI filter assembly employs a novel combination of three stages to produce a polarization-encoded interference pattern. Each stage of the filter performs one of the operations in the method. In the first stage of the OBPI filter assembly, the input image is split into two orthogonally polarized secondary eigen-images and one eigen-image is retarded relative to the other in a controlled, deterministic, spatially-varying manner by what will be called a first modified Babinet system. In the second stage, each secondary eigen-image is further split into two orthogonally polarized tertiary eigen-images and one eigen-image is retarded relative to the other in a controlled, deterministic, spatially-varying manner by what will be called a second modified Babinet system. In the third stage, a polarizer preferentially passes that component of each tertiary eigen-image that is substantially identical to the major eigenpolarization state of the polarizer, thus allowing the four tertiary eigen-images to interfere. The resulting polarization-encoded interference pattern identifies the complete state of polarization across the original partially-polarized image.
In a quantitative embodiment of the invention described herein, an electronic detector array performs a spatial discretization of the continuous interference pattern generated by the OBPI filter. Analog-to-digital conversion of the pixel illumination levels contained within the electronic image provides a set of raw data. Applying a novel mathematical reconstruction algorithm to the data allows for the computation of the polarimetric parameters of interest, i.e., the Stokes parameters, across the image in a no-moving-parts instantaneous manner.
It is therefore a principal object of the present invention to provide a novel D interferometric polarization measurement method by which polarization information contained within an arbitrary wavefront ensemble of interest is uniquely encoded within an interference pattern.
It is another object of the present invention to provide a novel interferometric polarization interrogating filter assembly that produces a polarization-encoded interference pattern uniquely representative of the spatially-varying, complete state of polarization across some arbitrary wavefront ensemble of interest.
It is a further object of the present invention to provide a novel mathematical reconstruction algorithm for decoding the polarimetric parameters of interest from the electronic image produced by detector array discretization of the polarization-encoded interference pattern.
It is a final object of the present invention to provide a system for simultaneously measuring all four Stokes parameters across an image comprised of a plurality of optical wavefronts in a no-moving-parts, instantaneous manner.
The imaging polarimeter instrument made possible by the method and assembly described herein has at least the following advantages over the prior art:
1. It is a complete polarimeter capable of simultaneously measuring all four Stokes parameters and hence the most general state of partial polarization across the image.
2. It requires no moving parts and measures the complete state of polarization instantaneously in a non-time-sequential manner.
3. It is a passive assembly that requires no additional electrical power besides that necessary to operate a single detector array and its associated electronic circuitry.
4. It can be employed over a substantial wavelength region of interest from the ultraviolet to the far infrared limited only by detection interrogating means.
5. It employs an internal polarization-encoding scheme that is completely independent of the dimensions and geometry of the detector array and, therefore, may be used with virtually any detector geometry.
6. It may be easily incorporated into preexisting conventional imaging instrumentation in one of several, non-permanent, ways.
7. It can be used, with suitable spectral interrogating means, as part of an imaging spectro-polarimeter instrument.
8. It is compatible with micro-lenses directly deposited onto individual pixel elements of the detector array to increase its fill factor.
9. It is compatible with interlaced color filter arrays utilized in digital color photography.
10. It can be used for both imaging polarimeters and point polarimeters.
11. It can be used by a trained observer to discern local variations in the state of polarization throughout a scene with the unaided eye.
12. It is straightforward to manufacture.
13. It requires no precise registration relative to the detector array provided that its orientation is held constant during calibration and image acquisition.
14. It may comprise a single, optically-bonded, assembly that is robust and insensitive to mechanical vibrations and temperature effects thereby being compatible with a hostile operating environment.
15. It can be used with narrowband or broadband light.
16. It may be mechanically small and minimally intrusive to preexisting imaging instrumentation.
17. It does not disturb the careful balance of aberration correction in preexisting imaging instrumentation.
18. It employs a novel reconstruction algorithm whose mathematical architecture enables computing the polarization parameters of interest in a quasi-real-time fashion.
19. It presents the same state of polarization to each pixel in the detector array regardless of the incident test wavefront ensemble and thus eliminates detector polarization sensitivity.
The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.